Process for providing gradual deployment of an airbag and a pyrotechnic charge for its implementation

ABSTRACT

The invention relates to the field of protecting the occupants of a motor vehicle by an inflatable airbag. 
     The invention relates to a process consisting in inflating an airbag (1) by a pyrotechnic generator (2) containing a double-composition charge (9) formed around a main charge having a linear burning rate V 1  and at least part of the ignition faces of which are covered with a skin of moderate pyrotechnic composition having a linear burning rate V 2  satisfying the relationship: 
     
         0.05 V.sub.1 ≦V.sub.2 ≦0.5 V.sub.1. 
    
     The invention thus makes it possible to inflate the airbag gradually during the first milliseconds of deployment of the latter.

The present invention relates to the field of protection, in the eventof an impact, of the occupants of a motor vehicle by means of an airbaginflated by a pyrotechnic gas generator. More specifically, theinvention relates to a process for ensuring gradual deployment of theairbag. The invention also relates to a pyrotechnic charge allowingimplementation of the process according to the invention.

It is known, in the event of a collision of a motor vehicle, to protectthe occupants of this vehicle by means of airbags which are inflated bythe gases arising from a pyrotechnic generator. The gases serving toinflate the airbag may come entirely from the combustion of apyrotechnic charge or result from mixing the gases arising from thecombustion of a pyrotechnic charge with gases stored in a pressurizedchamber, as is the case for so-called "hybrid" gas generators. Aspyrotechnic gas-generating compositions, those skilled in the art mayuse inorganic compositions based on sodium azide such as thosedescribed, for example, in French Patent FR-B-2,649,478 or in itscorresponding American patent U.S. Pat. No. 5,139,280. Those skilled inthe art may also use compositions similar to the propellent powders forweapons, having a single base comprising nitrocellulose or two basescomprising nitrocellulose and nitroglycerine, or else so-called"composite" powders based on an organic binder and an oxidizing charge,as described, for example in French Patent FR-B-262,322 or in itscorresponding American patent U.S. Pat. No. 5,160,163.

Airbags for protecting the occupants of a motor vehicle must be able tobe deployed in an extremely short time, of the order of 30 to 40 ms, andrequire, in order to do this, the use of pyrotechnic charges having veryhigh burning rates. However, the use of such charges poses a new problemsince too sudden a deployment of the airbag in the first milliseconds ofoperation may cause serious injuries to the vehicle's occupant who theairbag is supposed to protect. It is therefore necessary to use a chargehaving an overall burning rate compatible with the requirements ofmotor-vehicle safety but having a gradual combustion law. Those skilledin the art say that, on firing in a closed chamber, the curverepresenting the pressure in the chamber as a function of time has, forsuch a charge, a general S-shaped appearance.

This type of consideration departs completely from the use ofpyrotechnic charges completely or partially covered with an ignitionenhancer coating. These charges, which are described, for example, inpatent U.S. Pat. No. 4,246,051, have an initial pressure developmentwhich is incompatible with the desired gradual behaviour.

In order to obtain gradual deployment of the airbag, those skilled inthe art have explored several types of solution.

Those skilled in the art have sought to manufacture charges comprisingseveral layers having different thicknesses and different burning rates.Patent U.S. Pat. No. 5,507,890 thus describes such sodium-azide-basedcharges produced by pressing. Nevertheless, the technique thus describedis limited in its possibilities of application to compositions based onsodium azide, which compositions are, moreover, increasingly forsakenbecause of the toxicity and safety problems associated with compositionsof this type.

Those skilled in the art have also sought to obtain gradual charges bycoating charges based on compositions having a high burning rate with aninhibiting skin based on a resin or on inert inorganic substances, asdescribed, for example, in Patent Applications EP-A-0,586,045 andEP-A-0,586,060. However, these solutions are not satisfactory from thestandpoint of the non-toxicity and cleanness of the gases thusgenerated.

Those skilled in the art have therefore sought to obtain a gradualemission of gases by using generators having multiple coaxial combustionchambers, each containing a different pyrotechnic charge. This solution,described for example in patent U.S. Pat. No. 5,529,335 which relatesmore particularly to a generator for a seat-belt retractor, involves acomplex structure of the generator and therefore a high cost of thelatter.

Those skilled in the art do not currently have available a simple andeconomical solution for providing gradual inflation, under the timeconditions imposed by motor-vehicle safety requirements, of an airbag.

The object of the present invention is specifically to provide asolution to this problem.

The invention therefore relates to a process allowing an airbag forprotecting an occupant of a motor vehicle to be inflated in a gradualmanner by means of a device consisting mainly of the said airbagconnected to a pyrotechnic generator of gases arising at least partiallyfrom the combustion of a solid pyrotechnic charge placed inside the saidgenerator, the combustion of which charge may be initiated by an ignitercoupled to a collision detector, characterized in that:

i) a solid pyrotechnic double-composition charge is used which consistsof a main pyrotechnic charge having a linear burning rate V₁ at leastequal to 25 mm/s under a pressure of 20 MPa and the surface of which isat least partially covered with an external skin of pyrotechniccomposition adhering to the said main charge by simple deposition andhaving a linear burning rate V₂ which satisfies the relationship:

    0.05 V.sub.1 ≦V.sub.2 ≦0.5 V.sub.1 ;

ii) the said double-composition charge is placed inside the saidgenerator so that the entire surface of the main charge thus coveredserves as the combustion initiation surface of the latter when the saidigniter is actuated by the collision detector.

The process according to the invention thus relates both to protectiondevices in which all of the gases serving to inflate the airbag arisefrom the combustion of a solid pyrotechnic charge and to those in whichthe gases which serve to inflate the airbag arise only partly from thecombustion of a solid pyrotechnic charge, the other part coming eitherfrom a pressurized gas container, as is the case for so-called "hybrid"generators, or even from the outside, as is the case for siphon systems.

According to a first essential characteristic of the invention, a solidpyrotechnic double-composition charge is used which consists of a maingas-generating pyrotechnic charge having a linear burning rate V₁ atleast equal to 25 mm/s and preferably close to 40 mm/s under anoperating pressure of 20 MPa in order to be able to meet themotor-vehicle safety requirements. This main charge may be in the formof discs, but advantageously it will be in the form of an annularaxisymmetric block having at least one central channel allowing it to bepositioned around the igniter. This block will also preferably compriseat least one peripheral channel and, in general, a plurality ofperipheral channels.

The surface of this main charge must be at least partially covered withan external skin of a pyrotechnic gas-generating composition whoseburning rate will be less than the burning rate of the main charge butwhich will not be zero. The inhibiting compositions, which degrade withheat but which do not have a proper burning rate, are thereforeexpressly excluded from the field of the present invention. In practice,a pyrotechnic composition will be chosen, in order to form this skin,which has a linear burning rate V₂ which satisfies, under the samepressure conditions, the relationship:

    0.05 V.sub.1 ≦V.sub.2 ≦0.5 V.sub.1

and preferably:

    0.1 V.sub.1 ≦V.sub.2 ≦0.5 V.sub.1.

This external skin of pyrotechnic composition must adhere perfectly tothe main charge without any pyrotechnic discontinuity and especiallywithout the use of adhesive. This skin must therefore adhere to the maincharge by simple deposition. For this reason, the external skin and themain charge will usually be of the same nature.

According to a first embodiment of the invention, nitrocellulose-basedpyrotechnic compositions will be used to constitute the said mainpyrotechnic charge and the said external skin. This is because thesecompositions lend themselves well to the preparation of lacquers whichcan be deposited by dip-coating or spraying as thin layers on the maincharge in order to constitute, after evaporation of the solvent, a skinwhich adheres perfectly to the said main charge. Advantageously, it willbe possible to incorporate powerful and non-powerful plasticizers intothe said lacquer. One particularly preferred solution in this contextconsists in using a main pyrotechnic charge consisting of double-basepowder comprising nitrocellulose and nitroglycerine, this main chargebeing covered with a skin of single-base powder comprisingnitrocellulose. However, for reasons of stability over time, accordingto a preferred second embodiment of the invention, composite pyrotechniccompositions mainly consisting of a binder and a powerful charge, suchas the compositions known by the name "LOVA" for example, will be usedto constitute the said main external charge and the said external skin.

Depending on the type of binder, it will be possible to deposit on themain charge, by dip-coating or spraying, the composition layer intendedto constitute the external skin either in the form of a solution in avolatile solvent or in the form of a composite paste whose binder is notpolymerized, its polymerization taking place after deposition on themain charge. In order to improve the adhesion of the pyrotechnic skin tothe main charge, composite compositions containing the same binder willadvantageously be used to constitute the said main charge and the saidpyrotechnic skin.

As particularly preferred pyrotechnic compositions in the context of thepresent invention, the compositions described in French PatentFR-A-2,728,562 or in its corresponding American patent U.S. Pat. No.5,610,444, or else those described in French Patent Application 2750422or its corresponding U.S. application Ser. No. 08/874,634, may bementioned.

According to a preferred embodiment of the invention, a main pyrotechniccharge will be used in the form of an annular block, only the externallateral surface of which is covered with a pyrotechnic skin of burningrate V₂.

According to a second characteristic of the invention, the saiddouble-composition charge is placed inside the said generator so that atleast the entire surface of the main charge thus covered serves as thecombustion initiation surface of the latter when the said igniter isactuated. It is in fact essential, in the context of the invention, thatthe external skin of slow pyrotechnic composition contribute entirely tothe combustion initiation of the charge and, to achieve this, it isessential that this entire skin be able to be reached by the hot gasesarising from the igniter. This means, in particular when thedouble-composition charge is in the form of an annular block, that thisblock must have dimensions smaller than those of the combustion chamberof the generator and that it must be wedged, inside this chamber, bywedging means permeable to the ignition gases. To do this, a pyrotechnicgenerator will advantageously be used which consists of a hollowcylindrical body in the form of a can, comprising a side wall and aplane face and closed by a closure ring through which a central ignitiondevice passes, this ignition device entering the said can and beingsurrounded by a pyrotechnic charge in the form of an annular block heldin place by wedging systems permeable to the gases, such as meshes andsprings for example, the side wall of the said generator moreover havinggas outlet orifices which are blocked off, before operation, by a thinmetal shell of copper internal to the said generator.

If the main charge includes regions not covered by the said pyrotechnicskin, these regions will be able, depending on the case, to contributeor not to the combustion initiation of the pyrotechnic charge.

The invention also relates to the pyrotechnic double-composition chargesallowing implementation of the process which has just been described.These charges consist of an encapsulated main charge having a linearburning rate at least equal to 25 mm/s under a pressure of 20 MPa andare characterized in that at least part of the surface of the said maincharge is covered with a skin of pyrotechnic composition adhering to thesaid main charge by simple deposition and having a linear burning rateV₂ which satisfies, under a pressure of 20 MPa, the relationship:

    0.05 V.sub.1 ≦V.sub.2 ≦0.5 V.sub.1.

As already mentioned above, the linear burning rate V₂ will preferablysatisfy the relationship:

    0.1 V.sub.1 ≦V.sub.2 ≦0.5 V.sub.1.

This skin may cover the entire surface of the main charge, including thesurface of the channels in the case of a multiperforated annular blockor only a part, for example only the external surface in the case of amain charge in the form of an annular block. As will now be explained indetail, the combustion of this skin must correspond to the entire firstmilliseconds of combustion of the complete twin-composition charge. Thisskin will therefore contain little pyrotechnic material and itsthickness will be very small, usually less than or equal to 0.1 mm andpreferably of the order of a few hundreds of a millimeter. In practice,it will be sought to set the thickness of this skin so that itscombustion time is at most equal to one-quarter of the combustion timeof the main charge.

Thus, in the event of a collision of the vehicle, the collision detectorsends an electrical signal to the igniter which is fired and sends hotgases over at least the entire surface of the twin-composition chargecovered with the skin of pyrotechnic composition with a slower burningrate. This produces a moderate evolution of gas which initiates thedeployment of the airbag. Since the thickness of this skin is verysmall, as indicated above, after a few milliseconds the skin hascompleted its own combustion and has transmitted the burning material tothe surfaces of the main charge which have not yet been initiated. Thecombustion then continues over the entire surface of the main charge ata high rate and a high gas flow rate, which continues and completes thedeployment of the airbag in the time required for motor-vehicle safety.The invention thus offers those skilled in the art the possibility ofgradually deploying the airbag using conventional generators and withoutappreciably increasing the manufacturing costs, the production of atwin-composition pyrotechnic charge by dip-coating or spraying beingeasily automatable.

One example of implementation of the invention, according to a preferredembodiment, is described below with reference to FIGS. 1 to 4.

FIG. 1 shows, in a partial sectional view, a device allowing the processaccording to the invention to be implemented.

FIG. 2 shows, seen in perspective, a twin-composition charge used in thecontext of the invention.

FIG. 3 shows, seen in cross-section in a plane of axial symmetry, thecharge shown in FIG. 2.

FIG. 4 shows pressure curves for a device according to the invention inoperation.

FIG. 1 shows a device for protecting an occupant of a motor vehicle,consiting of an inflatable airbag 1 fixed to a pyrotechnic gas generator2 by screws 3 which also serve to fix the device to the vehicle.

The generator 2 consists of a hollow cylindrical body in the form of acan comprising a side wall 4 and a plane wall 5 made as a single piece.This can is closed by a closure ring 6 crimped in the free end of thecan constituting the body of the generator. The closure ring 6 has acentral orifice flanged with a neck 7 entering the generator 2. Anelectrical igniter 8 is crimped in the neck and is coupled by anelectrical connection to a collision detector, not shown in the figure.A pyrotechnic charge 9 in the form of an annular block is placed insidethe generator around the igniter 8. Referring more particularly to FIG.2, it may be seen that this annular block has the shape of a lobate ringhaving a central large-diameter channel 10, each lobe 11 having asmall-diameter channel 12. The dimensions of the charge 9 are less thanthose of the internal volume of the generator 2, the charge being heldin place by assemblies 13 and 14 which are permeable to the gasesarising from the igniter 8.

The side wall 4 of the body of the generator 2 has gas outlet orifices15 arranged near the plane wall 5. These orifices 15 are capped by avery thin copper internal can 16, the free end of which is pinchedbetween the closure ring 6 and the side wall 4.

These orifices 15 are arranged opposite a perforated deflector 17carried and held in place by an external annular ring 18 and by anannular diffuser 19. The external annular ring 18 rests on an externalflat 22 on the side wall 4, while the diffuser 19 is crimped in the rimof the plane wall 5. Filtering meshes 20 and 21 also wedged in betweenthe external ring 18 and the diffuser 19 complete the gas outlet devicewhich is inside the inflatable airbag 1.

Referring now more particularly to FIG. 3, it may be seen that thepyrotechnic charge 9 is composed of a main charge 23 having the geometrydescribed above and the external lateral surface of which main charge iscovered with an external skin 24 whose thickness has been exaggeratedfor the sake of clarity in FIG. 3, as is also the case in FIG. 1.

The main charge 23 is a composite charge essentially consisting of asilicone binder and an oxidizing charge which is an ammoniumperchlorate/sodium nitrate mixture, according to the technique describedin patent U.S. Pat. No. 5,610,444. The oxidizing-charge content in thismain charge 23 was 80 parts by weight. The external skin 24 consists ofa pyrotechnic composition of the same nature, in which theoxidizing-charge content was 40 parts by weight only. This skin 24 wasdeposited on the main charge 23 by spraying before complete crosslinkingof the silicone binder. The thickness of the skin 24 was less than 0.1mm (one-tenth of a mm) and its linear burning rate V₂ at 20 MPa was ofthe order of 4 mm/s for a linear burning rate V₁ at 20 MPa of 40 mm/sfor the main charge 23.

The operation of this generator is as follows. In the event of anaccident, the collision detector, not shown in the figures, sends anelectrical signal which fires the igniter 8 which sends hot gases inorder to initiate the combustion of the charge 9 over all of itsaccessible surfaces. It should be pointed out that the plane surfaces ofthis charge, as well as the internal surfaces of the central channel 10and of the peripheral channels 12 burn immediately at the high rate V₁,while the side surface of the charge 9 starts to burn at the moderaterate V₂ so as to burn at the high rate V₁ only when the thickness of theskin 24 has been burned.

When the pressure inside the generator reaches a predeterminedsufficient value, the caps consisting of the parts of the can 16 whichlie opposite the orifices 15 burst and the combustion gases, distributedby the perforated deflector 17, pass through the filtering meshes 20 and21 before inflating the airbag 1. It should be pointed out that, in thecontext of the present invention, the caps must burst before the skin 24has finished burning.

Surprisingly, it has been observed that the use of a thin copperinternal can 16 makes it possible to obtain cleaner and less toxic gasesthan those obtained with a conventional thin aluminium internal can.

By way of example, a pyrotechnic generator identical to the one justdescribed was operated in a 60-liter sealed chamber, representing thevolume offered by an airbag. The pressure P_(C) inside the generator, onthe one hand, and the pressure P_(T) in the sealed chamber, on the otherhand, were measured using pressure sensors. FIG. 4 shows the curvesP_(C) and P_(T) thus obtained, as a function of time. Curve P_(T)clearly illustrates the gradual nature of the pressure rise in thesealed chamber.

What is claimed is:
 1. Process allowing an airbag (1) for protecting anoccupant of a motor vehicle to be inflated in a gradual manner by meansof a device consisting mainly of the said airbag connected to apyrotechnic generator (2) of gases arising at least partially from thecombustion of a solid double-composition pyrotechnic charge (9) placedinside the said generator, the combination of which charge may beinitiated by an igniter (8) coupled to a collision detector,characterized in that:(i) a solid pyrotechnic charge is used whichconsists of a main pyrotechnic charge (23) having a linear burning rateV₁ at least equal to 25 mm/s under a pressure of 20 MPa and the surfaceof which is at least partially covered with an external skin (24) ofpyrotechnic composition adhering to the said main charge by simpledeposition, the thickness of which being less or equal to 0.1 mm andhaving a linear burning rate V₂ which satisfies, under a pressure of 20MPa, the relationship:

    0.05 V.sub.1 ≦V.sub.2 ≦0.5 V.sub.1 ;

(ii) pyrotechnic nitrocellulose-based compositions and compositepyrotechnic compositions mainly consisting of a binder and of a powerfulcharge are used to constitute the said main pyrotechnic charge and thesaid pyrotechnic skin; (iii) the said double-composition charge isplaced inside the said generator so that the entire surface of the maincharge thus covered serves as the combustion initiation surface of thelatter when the said igniter is actuated by the collision detector. 2.Process according to claim 1, characterized in that the linear burningrate V₂ of the pyrotechnic skin (24) satisfies, under a pressure of 20MPa, the relationship:

    0.1 V.sub.1 ≦V.sub.2 ≦0.5 V.sub.1,

V₁ being the linear burning rate, under a pressure of 20 MPa, of themain charge.
 3. Process according to 1, characterized in that apyrotechnic generator (2) is used which consists of a hollow cylindricalbody in the form of a can, comprising a side wall (4) and a plane face(5) and closed by a closure ring (6) through which a central ignitiondevice (8) passes, this ignition device entering the said can and beingsurrounded by a pyrotechnic charge in the form of an annular block (9)held in place by wedging systems (13, 14) permeable to the gases, theside wall of the said generator having gas outlet orifices (15) whichare blocked off by a thin metal shell (16) of copper internal to thesaid can.
 4. Process according to claim 1, characterized in thatcomposite compositions containing the same binder are used to constitutethe said pyrotechnic charge and the said pyrotechnic skin.
 5. Processaccording to claim 1, characterized in that a charge in the form ofdiscs is used as the main pyrotechnic charge.
 6. Process according toclaim 1, characterized in that a charge in the form of an annular blockhaving at least one central channel is used as the main pyrotechniccharge.
 7. Process according to claim 6, characterized in that only theexternal lateral surface of the said annular block is covered with apyrotechnic skin (24).